Loeb and Sourirajan disclosed a method for preparing a modified cellulose acetate membrane for desalination of water by first casting a solution of cellulose acetate as a thin layer, and then forming a very thin dense membrane skin on this layer through various techniques such as solvent evaporation followed by quenching in cold water. The formation of these thin skinned membranes generally involved a final annealing treatment in hot water. The membranes prepared by the Loeb-Sourirajan method are composed of two distinct regions made of the same cellulose acetate material, a thin, dense, semi-permeable skin and less dense, void-containing, non-selective support region. Since the membranes are not of essentially the same density throughout their structure, they fall within the definition of anisotropic membranes. Because of these distinct regions and the difference in membrane properties which can be observed depending on which surface of the membrane faces a fluid feed mixture, the Loeb-Sourirajan membranes can be described as being asymmetric.
In, for instance, practical desalination tests, asymmetric, thin dense-skinned membranes have been shown to have superior permeate flux when compared to the older style thick dense membranes. The improvement in the permeation rate of the Loeb-Sourarijan membranes has been attributed to the decrease in the thickness of the dense selective region. The less dense region in such a membrane provides sufficient structural support to prevent rupture of the membrane under operating pressures, but offers little resistance to permeate flow. Hence, the separation is essentially accomplished by the dense skin, and the primary function of the less dense support region is to physically support the dense skin. However, in such Loeb-Sourarijan membranes this less dense support region is frequently compacted by pressures such as those desirable for desalination of water and under such conditions the less dense support region loses some of its void volume. Consequently, the free flow of permeate away from the effluent side of the dense skin is hindered, resulting in a reduced permeation rate. Therefore, attention has been directed to developing Loeb-Sourirajan membranes of materials other than cellulose acetate which may provide stronger structural properties and increased chemical resistance. The development of asymmetric membranes from hydrophobic polymer materials to obtain a single component membrane exhibiting good selectivity and a good permeation rate has been found to be extremely difficult. Most attempts result in producing membranes which are either porous, i.e., have fluid flow channels through the dense skin, and will not separate, or which have too thick a dense skin to give useful permeation rates. Thus, those asymmetric membranes often fail to meet with acceptance in liquid separation operations such as reverse osmosis. As hereinafter further described, it is even more difficult to provide Loeb-Sourirajan membranes which exhibit good selectivity and permeation rates for gas separation operations.
The above discussion of the background to this invention has been directed to membranes for the separation of a liquid from a liquid mixture such as in desalination of water. More recently emphasis has been placed on developing separation membranes which are suitable for separating a gas from a gaseous mixture. The permeation of gases through separation membranes has been the subject of various studies. The following discussion is illustrative of specific modifications which have been made to membranes used for gas separations and provides a basis upon which this invention can be fully appreciated.
Attempts have been made to draw on knowledge developed concerning liquid-liquid separation membranes. There are, however, many different considerations in developing a suitable separation membrane for gaseous systems as compared to developing a suitable membrane for liquid systems. For instance, the presence of small pores in the membrane may not unduly adversely affect the performance of the membranes for liquid separations such as desalination due to absorption on and swelling of the membrane and the high viscosity and the high cohesive properties of the liquids. Since gases have extremely low absorption, viscosity and cohesive properties, no barrier is provided to prevent the gases from readily passing through the pores in such a membrane resulting in little, if any, separation of gases. An extremely important difference between liquids and gases which might affect selective separation by permeation through membranes is the generally lower solubility of gases in membranes as compared to the solubility of liquids in such membranes, thus resulting in lower permeability constants for gases as compared to those of liquids. Other differences between liquids and gases which might affect selective separation by permeation through membranes include density and internal pressure, the effect of temperature on the viscosity, surface tension, and the degree of order.
It has been realized that materials which exhibit good separation of gases often have lower permeability constants compared to those of materials exhibiting poor separation of gases. In general, efforts have been directed to providing the material of a gas separation membrane in as thin a form as possible, in view of the low permeabilities, in order to provide adequate flux yet provide a membrane as pore-free as possible, such that gases are passed through the membrane by interaction with the material of the membrane. On approach to developing separation membranes suitable for gaseous systems has been to provide composite membranes having a superimposed membrane supported on an anisotropic porous support wherein the superimposed membrane provides the desired separation, i.e., the superimposed membrane is semipermeable. The superimposed membranes are advantageously sufficiently thin, i.e., ultrathin, to provide reasonable fluxes. The essential function of the porous support is to support and protect the superimposed membrane without harming the delicate, thin superimposed membrane. Suitable supports provide low resistance to permeate passage after the superimposed membrane has performed its function of selectively separating the permeate from the feed mixture. Thus, these supports are desirably porous to provide low resistance to permeate passage and yet sufficiently supportive, that is, having pore sizes sufficiently small to prevent the rupture of the superimposed membrane under separating conditions. Klass, et al., U.S. Pat. No. 3,616,607, Stancell, et al., U.S. Pat. No. 3,657,113 and Yasuda, U.S. Pat. No. 3,775,303 exemplify gas separation membranes having superimposed membranes on a porous support.
Such composite membranes for gas separations have not been without problems. For instance, Browall in U.S. Pat. No. 3,980,456 discloses the fabrication of composite membrane films for separation of oxygen from air comprising a support of microporous polycarbonate sheet and separately formed, i.e., preformed, superimposed ultrathin separation membrane of 80 percent poly(phenylene oxide) and 20 percent organopolysiloxane-polycarbonate copolymer. In the fabrication of the membranes the exclusion from the manufacturing area of extremely small particulate impurities, i.e., particles below about 3000 angstroms in size, is stated by Browall to be impractical or impossible. These fine particles may be deposited under or between preformed ultrathin membrane layers and, because of their large size in comparison to the ultrathin membranes, puncture the ultrathin membranes. Such breaches reduce the effectiveness of the membrane. The Browall patent discloses applying a preformed organopolysiloxane-polycarbonate copolymer sealing material over the ultrathin membrane to cover the breaches caused by the fine particles. Browall also discloses employing a preformed layer of the organopolysiloxane-polycarbonate copolymer between the ultrathin membrane and the porous polycarbonate support as an adhesive. Thus, the composite membranes of Browall are complex in materials and techniques of construction.
In summary, apparently suitable anisotropic membranes have not been provided for gas separations which, in the absence of a superimposed membrane to provide the selective separation, exhibit sufficient flux and selectivity of separation for general commercial operations. It further appears that composite membranes for gas separation which have a superimposed membrane to provide the selective separation have achieved only slight or modest improvement in membrane performance. Moreover, the superimposed membrane, although possibly ultrathin in order to provide the desired selectivity of separation, may significantly reduce the flux of the permeate gas through the composite membrane as compared to that of the porous support not having the superimposed membrane thereon.
The aforementioned difficulties with gas separation membranes were further addressed by Henis and Tripodi and resolved as taught in their U.S. Pat. No. 4,230,463, herein incorporated by reference, whereby a multicomponent membrane for gas separation comprised of a coating in contact with a porous separation membrane was taught with the separation properties of the multicomponent membranes being principally determined by the material of porous separation membrane as opposed to the membrane material of the coating. The multicomponent membranes provided gas separation membranes which could be fabricated from a wide variety of gas separation materials and thus enable greater latitude in selecting such a membrane material which is advantageous for a given gas separation. These multicomponent membranes were solvent spun from solvent, solvent/non-solvent dopes of glassy, aromatic sulfone polymers and other polymers and coagulated in water and other polar media. Separation factors for at least one pair of gases were increased by at least 35 percent through utilization of the multicomponent membranes; however, separation factor and permeability remain coupled, that is, adjustment or increase in, for example, separation factor would be at the cost of permeation rate and vice versa.